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The Class Bivalvia - by way of the incurrent siphon and to...

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The Class Bivalvia The class Bivalvia consists of clams, oysters, mussels, and scallops. Members of this class have two-part shells that are hinged and closed by powerful muscles. The presence of shells in this group has yielded an impressive fossil record. The bivalves have no head, no radula, and little cephalization , as can be seen in Figure 5. Clams use their hatchet-shaped foot for burrowing; mussels use it to produce threads to attach to objects. Scallops can both burrow or swim. A rapid closing and opening of their two valves releases water in spurts. The bivalve shell is secreted by the mantle. The shell is composed of protein and calcium carbonate with an inner layer of pearl. Pearls form as layers of shell-forming material deposited about a foreign particle lodged between the mantle and the shell. A compressed muscular foot projects down from shell. By expanding the tip, the foot pulls the body after it. Beating cilia of the gills cause water to enter the mantle cavity
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Unformatted text preview: by way of the incurrent siphon and to exit by way of the excurrent siphon. While cilia of gills move water through the mantle cavity, gills also capture particles in water and move them toward the mouth. From the mouth food goes to the stomach, then to the intestine, which passes through the heart and ends at the anus. Bivalves, like other mollusks, have an open circulatory system. Their nervous system consists of three pairs of ganglia. Two excretory kidneys below the heart remove ammonia waste from the pericardial cavity into the mantle cavity, from which it will leave the body. Sexes in class Bivalvia are separate. The gonad is located around the coils of the intestine. Certain clams and annelids have the same type of larva, hinting at a possible evolutionary relationship between the two groups....
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